In the solar cell, carriers (electrons and holes) generated by light irradiation on a photoelectric conversion section having a semiconductor junction are extracted to an external circuit to generate electricity. A collecting electrode is provided on the photoelectric conversion section of the solar cell for efficiently extracting carriers generated at the photoelectric conversion section to the external circuit. For example, in a crystalline silicon-based solar cell using a single-crystalline silicon substrate or a polycrystalline silicon substrate, a collecting electrode made of a fine metal is provided on a light-incident-surface. Also, in a heterojunction solar cell having amorphous silicon layers and transparent electrode layers on a crystalline silicon substrate, a collecting electrode is provided on the transparent electrode layer.
The collecting electrode of the solar cell is generally formed by pattern-printing a silver paste (electroconductive paste) by a screen printing method. Regarding a silver paste applicable for formation of a collecting electrode for a solar cell, The following patent documents describe the use of an electroconductive paste containing a spherical electroconductive filler (granular metal particles) and a flaky electroconductive filler (flaky metal particles), both for a reduction of the line width (fining) of a collecting electrode, reduction of resistance, improvement of adhesive strength, and so on: JP A 2002-76398, JP A 2010-87131, and JP A 2013-196954.
One problem with screen printing a silver paste is that the material cost of silver is high. Another problem is that the resistivity of a collecting electrode increases because the silver paste contains an insulating resin material used as a binder. To reduce the resistance of a collecting electrode formed using a silver paste, it is necessary to thickly print the silver paste. Increasing the printing thickness causes a line width of the electrode to increase. Therefore, it is difficult to decrease the collecting electrode width, so that a shading loss caused by the collecting electrode easily increases.
To solve these problems, a method is known for forming a collecting electrode by a plating method that is excellent in terms of a material cost and a process cost. JP A 2013-507781 discloses a method in which as an underlying layer for plating, an electroconductive seed having a large surface roughness is provided on a transparent electrode layer, and an insulating layer is formed on the electroconductive seed. Specifically, an electroconductive seed is formed on a transparent electrode layer using an electroconductive paste containing silver flakes, nanoparticles or the like, and then an insulating layer is formed. The transparent electrode layer is covered with the insulating layer except for an electroconductive seed-formed section. A discontinuous opening section is formed in the insulating layer on the electroconductive seed due to the surface roughness of the electroconductive seed. A metal layer is formed on the electroconductive seed through the opening section of the insulating layer by an electroplating method.
WO 2013/077038 discloses a solar cell including a transparent electrode layer, a first electroconductive layer (plating underlying layer) and a second electroconductive layer (plating layer) on the transparent electrode layer, and an insulating layer between the first electroconductive layer and the second electroconductive layer. WO 2013/077038 describes that an insulating layer is formed on a first electroconductive layer containing a low-melting-point material. An annealing treatment is then performed by heating to form an opening section in the insulating layer on the first electroconductive layer due to thermal-fluidization of the low-melting-point material. A second electrconductive layer is formed on the first electroconductive layer through the opening section by a plating method. WO 2013/077038 also describes that metal particles having a particle size of 1 μm or less can be applied as a low-melting-point material because thermal-fluidization associated with fusion occurs at a temperature lower than a melting point.